The invention relates to a process for producing low-viscosity, aqueous starch dispersions containing starch granules that are gelatinized but are unpasted, that is, do not become highly viscous.
An important technological property displayed by starch is its great swelling capacity. However, swelling is generally followed by coagulation into a starch paste. Swelling is brought about by heating, but also by an enzymatic treatment or by adding an alkali at room temperature.
Dispersions that do not form pastes under these conditions are needed in the case where starch is to be chemically modified by esterification, etherification, oxidation or by some other reaction. The high viscosity of typical starch pastes interferes with the chemical modification of the starch. Pastes having starch concentrations of 15% are, in part, no longer stirrable. Therefore, in order to provide adequate mixing with the reaction components, it is necessary to perform the reaction in a relatively high dilution.
Alternatively, by starting with a suspension of unswelled starch granules, the reaction can be conducted at a high starch concentration and low viscosity. In order to safely avoid pasting, however, a temperature of 50.degree. C. must not be substantially exceeded in this process. Reactions in such heterogeneous systems, though, frequently proceed unsatisfactorily since the reaction takes place only on the surface of the solid, unswelled particles. Thus, only minor degress of substitution are achieved.
A number of processes are known for the preparation of thinly fluid starch pastes, but these require a considerable interference with the structure of starch. For example, by partial hydrolytic breakdown with the aid of acids, a thin-boling starch is otained (Ullmanns Encyklopaedie der Technischen Chemie 22: 192 [1982]), yielding thinly fluid pastes when boiled in water. However, the pastes undergo secondary thickening upon cooling.
For the production of swelling starches, aqueous starch suspensions are heated, in most cases on drum dryers, past the pasting temperature and, in this shape, disintegrated, pasted, and dried. Thus swelling starches, which are also called cold-water-soluble starches, form pastes with water at room temperature. For this reason, they are not suitable for chemical reactions.
Polyhydroxy compounds, such as glucose or sucrose, draw water from starch. Thus, when heating a starch suspension after adding sugar, a slowed-down gelatinization and a slightly lowered viscosity of the paste are observed (Encyclopedia of Polymer Science and Technology 12: 822 [1970]). Similar effects are also disclosed by B. J. Oosten (Staerke [Starch] 36: 18 [1984]). By addition of sorbitol or glycerol, the gelatinization temperature determined by means of a polarizing microscope is increased by up to 14.degree. C. This temperature elevation is undesirable because starch reactions proceed incompletely below the gelatinizing temperature.
The gelatinizating temperature T.sub.gel is defined for the purposes of this disclosure as the temperature at which 50% of the starch granules have lost their birefringence (B. J. Oosten, Staerke, 36: 18 [1984]), which disclosure is incorporated by reference herein.
Pasting behavior is detected by measuring viscosity as a function of time and of temperature in the form of a so-called viscogram, wherein a starch slurry is heated at a certain velocity under agitation to a temperature of about 90.degree. C., maintained at this temperature for a specific time period (for example 30 minutes), whereupon the starch paste is cooled at a specific rate. The pasting temperature, T.sub.paste, is the temperature at which the dispersion, during heating, passes through a viscosity maximum (cf. Ullmanns Encyklopaedie der Technischen Chemie 22: 172 [1982]), which disclosure is incorporated by reference herein. During cooling of such a paste, the viscosity rises again and reaches a second maximum indicatiang relative solidity of the starch gel. The viscosity curves show a characteristic course for each type of starch. For example, pasting of potato starch leads to a relatively high viscosity of the paste which drops again greatly upon cooling. Cornstarch has a lower viscosity peak.
By heating an aqueous suspension of "native", i.e., naturally occurring strachm, gelatinization is immediately followed by coagulation into a starch paste. If it were possible to raise the pasting temperature once gelatinizing has occurred, then a temperature range would result within which a low-viscosity (i.e., unpasted), gelatinized starch dispersion would exist which would be readily suitable for chemical reactions.
According to O. Harbitz (Staerke 35: 198 [1983]), which disclosure is being incorporated by reference herein, addition of dodecanoic acid raises the pasting temperature by 2.degree. C. In contrast thereto, by adding sodium dodecyl sulfate, an anionic tenside, the pasting temperature of potato starch is lowered.
According to Japanese Pat. No. 72-45,938, which disclosure is being incorporated by reference herein, cationic and nonionic tensides, such as, for example polypropylene glycol-polyethylene glycol ethers, also cause lowering of the pasting temperature and accelerate pasting. In one example, the pasting temperature is lowered by 7.degree. C. by dodecyltrimethylammonium chloride.
Consequently, a person skilled in the art with the objective of producing low-viscosity dispersion of native starch that pastes only at an elevated temperature would expect that tensides would lower the pasting temperature and would be unsuitable additives in starch modification reactions.